EP0543752A1 - Process for preparing metal carbides of high specific surface from activated carbon foams - Google Patents

Abstract

Metal carbide or silicon carbide foam intended to be used as catalyst or catalyst support for the chemical or petrochemical industry or for exhaust systems, and process for its manufacture. The foam takes the form of a three-dimensional network of interconnected cages in which the length of the ridges is between 50 and 500 micrometers, whose density is between 0.03 and 0.1 g/cm<3> and whose BET surface is between 20 and 100 m<2>/g. The carbide foam contains no more than 0.1 % by weight of residual metal and the size of the carbide crystallites is between 40 and 400 angstroms. The process of manufacture consists, starting with a carbon foam, in increasing its specific surface by a treatment of activation with carbon dioxide and in finally bringing the foam thus activated into contact with a volatile compound of the metal whose carbide it is intended to obtain.

Description

TECHNICAL FIELD OF THE INVENTION.

The invention relates to the technical field of silicon carbides or heavy metals with a large specific surface intended to serve as catalysts or catalyst supports for the chemical or petrochemical industry or for exhaust pipes.

PROBLEM.

The specific surface of a catalyst is due to three types of porosity: a macroporosity due to pores with an average diameter greater than 2 μm, a mesoporosity due to pores with an average diameter of 30-50 angstroms and a microporosity due to pores of average diameter 5-15 angstroms. A catalyst must have sufficient macroporosity to give the gases to be treated access to the micro- and especially mesopores responsible for the catalytic activity proper.

French patent application No. 2,645,143 describes a process for preparing heavy metal carbides with a large specific surface which consists in reacting a compound of the heavy metal under vacuum, at a temperature such that this compound is gaseous, on carbon having a large specific surface, for example activated carbon. The carbide obtained retains the memory of the specific surface of the starting carbon and has a specific surface clearly greater than that of the metal carbides of the prior art, although less than that of the starting carbon.
In this process for the preparation of carbides, the catalysts or supports obtained are in the form of powder, or agglomerates of granule type intended to constitute a fixed bed placed in a reactor. The gases to be treated circulate, sometimes at high speed through this bed and this results in attrition phenomena with the production of fines which plug the bed or seal the sintered parts which delimit the bed.

For certain catalytic applications, in particular for the catalysis of the oxidation of exhaust gases, it is necessary to choose the geometry of the support or of the catalyst and to produce it in the form of monolithic parts, ensuring the accessibility of the catalyst to gas to be treated by its macroporosity.

The inventors posed the problem of obtaining carbides with a high specific surface in monolithic form and having a large open macroporosity allowing easy access to the reaction gases.

Thus, while in the process of French patent application No. 2,645,143, the carbide is first manufactured and then placed in a bed, in the process of the invention, a porous solid of carbon which is then transformed into metallic carbide.

OBJECT OF THE INVENTION.

The object of the invention is a process for the preparation of metal carbides with a high specific surface in monolithic form having a large open macroporosity allowing easy access to the reaction gases, characterized in that one starts with a carbon foam, which 'It is activated to increase its specific surface area and the activated foam is subjected, under inert gas sweeping or under vacuum, to the action of a volatile compound of the metal considered. The use of certain organic precursors making it possible by calcination to obtain carbon foams having interesting characteristics is also part of the invention.

A second object of the invention is a monolithic catalyst or catalyst support consisting of a foam of metallic carbide or silicon carbide having a large open macroporosity, a large specific surface due mainly to its mesoporosity and a small dimension of crystallites and containing no metal or other residual metallic compound.

DESCRIPTION OF THE PRIOR ART.

French patent application No. 2,645,143 cited above describes the principle of preparation of metal carbides with a large specific surface area as well as examples of implementation. The carbon with a large specific surface, for example activated carbon, is mixed with a compound, volatile at the reaction temperature, of the metal from which the carbide is to be obtained. The assembly placed in a crucible is then brought to a temperature of between 900 and 1400 ° C. under a vacuum of less than 13 hPa for a period of between 1 and 7 hours but most often of at least 4 hours.
As a variant, it is also possible to vaporize the volatile compound from the metal in a first zone and to carry out the reduction and the carburetion in a second zone, the temperatures of the two zones possibly being different.

• génération dans une première zone, de vapeur de SiO par réaction de SiO2 sur Si à une température comprise entre 1100 et 1400°C, sous un vide de 0,1 à 1,5 hPa;
• réaction, dans une deuxième zone, du SiO formé avec du carbone à surface spécifique supérieure à 200 m2/g, à une température comprise entre 1100 et 1400°C.

French patent application FR 2 621 904 describes the preparation of silicon carbide SiC with a specific surface greater than 100 m2 / g by:

• generation in a first zone, of SiO vapor by reaction of SiO2 on Si at a temperature between 1100 and 1400 ° C, under a vacuum of 0.1 to 1.5 hPa;
• reaction, in a second zone, of SiO formed with carbon with a specific surface greater than 200 m2 / g, to a temperature between 1100 and 1400 ° C.

US Patent 2,431,326 (Heyroth) describes a product consisting of a continuous and crosslinked backbone of crystallized SiC uniformly distributed and the interstices of which are filled with silicon or a compound rich in silicon.
The skeleton can be made by calcining wood, molasses, casein, dextrin, flour or other carbonizable substances. Preferably, the reaction of ClH or SO4H2 is used on furfural, furfuryl alcohol or their mixtures: very quickly the carbon precipitates and grows in dendritic form to form a sponge filling the entire container.
The skeleton thus obtained is brought into contact with silicon, for example by causing it to float on molten Si far above its melting point or by subjecting it to silicon vapors.

Patent FR 1 484 116 (US Atomic Energy Commission) describes a process for the preparation of carbon foams and carbides (in particular Si) by a technique derived from the preparation of polyurethane foams.
The reaction of an isocyanate on an alcohol or a phenol leads to a urethane and with diisocyanates and dialcohols, linear polyurethanes of formula are obtained:

(-OC-NH-R-NH-CO-OR′-O-)

In addition, isocyanates react with water to give CO2 and an ureine.

Polymerization in the presence of water therefore makes it possible to form polyurethane foams which, by carbonization, give carbon foams. However, the carbon yield is low and the foams obtained are not solid.

• d'une part à mélanger un liant dont le rendement en C est élevé: de l'alcool furfurylique partiellement polymérisé,
• d'autre part à mélanger des poudres de métaux ou d'oxydes métalliques pour obtenir des mousses métalliques ou de carbures métalliques ( W, Ta, Nb, Al, B, Si ).

The invention of this patent therefore consists of:

• on the one hand, to mix a binder with a high C yield: partially polymerized furfuryl alcohol,
• on the other hand to mix powders of metals or metal oxides to obtain metallic foams or metallic carbides (W, Ta, Nb, Al, B, Si).

In the process described, the carbide is obtained in a single operation, at the time of the calcination of the mixture of resins with the metal oxide or the metal, which does not make it possible to control the specific surface of the carbide obtained. In the process of the present patent application, on the contrary, the carbide is obtained in two separate stages by the activation of the foam which makes it possible to reach a much higher specific surface of the carbide (20 to 100 m² / g), a smaller crystallite dimension (40 to 400 angstroms = 40 to 400.10⁻¹⁰ m) and a carbide foam containing little or no residual metal (<0.1% by weight).

Patent GB 2 137 974 (UK Atomic Energy Authority) describes the manufacture of a silicon matrix reinforced by a network of silicon carbide by treatment of a carbon foam by contacting molten silicon.

DESCRIPTION OF THE INVENTION.

• de façon facultative, préparer de la mousse de carbone par calcination d'une mousse d'un polymère organique;
• fabriquer, à partir de la mousse de carbone ainsi préparée ou d'une mousse de carbone du commerce une pièce ayant les dimensions et la forme du support de catalyseur ou du catalyseur que l'on veut obtenir;
• de préférence, dégazer la pièce en mousse de carbone sous vide ou sous balayage d'un gaz inerte tel l'argon;
• activer la surface de cette pièce en l'exposant à une oxydation ménagée par le dioxyde de carbone CO2;
• exposer la pièce ainsi activée à l'action d'un composé volatil du métal dont on veut préparer le carbure à une température (1000 à 1400°C) et pendant une durée (2 à 6 heures) telles que le composé soit réduit en métal, et que le métal soit carburé. Cette étape est conduite soit sous vide, soit sous balayage de gaz inerte à la pression atmosphérique.

The method of the invention consists of the combination of the following successive steps:

• optionally, preparing carbon foam by calcining a foam of an organic polymer;
• manufacture, from the carbon foam thus prepared or from a commercial carbon foam, a part having the dimensions and the shape of the catalyst support or of the catalyst that it is desired to obtain;
• preferably degas the carbon foam part under vacuum or under sweeping of an inert gas such as argon;
• activate the surface of this part by exposing it to mild oxidation by carbon dioxide CO2;
• expose the part thus activated to the action of a volatile compound of the metal whose carbide is to be prepared at a temperature (1000 to 1400 ° C) and for a period (2 to 6 hours) such that the compound is reduced to metal , and that the metal is carburetted. This step is carried out either under vacuum or under sweeping of inert gas at atmospheric pressure.

Starting materials: Carbon foam

Carbon foams are in the form of a three-dimensional network of kinds of interconnected cages whose edges have thicknesses of the order of a few micrometers, the length of these edges being able to vary from 50 to 500 micrometers. Their BET surface is very small: it is most often less than 1 m² / g. Their density can vary in large proportions: from 0.03 to 0.1 g / cm³, depending on the relative dimensions of the thicknesses and lengths of the edges. Depending on the nature of the catalytic reaction and on the mechanical or thermal characteristics expected from the catalyst or its support, it will be necessary to choose foams of greater or lesser density. They are obtained by calcination of foams of organic resins, for example polyurethanes.

It is possible to obtain them commercially. However, the applicant has found that it was advantageous to prepare them from certain organic precursors, polymer foams, with a view to obtaining a porous texture favorable to their use as a support for catalysts or catalysts.

Two precursors are particularly well suited to the intended use: polyacrylonitrile or PAN and polyurethane foams.

The PAN used is in the form of rigid panels with a density of between 0.03 and 0.3 g / cm3, of the type sold for example by the company SAITEC. Its porous texture is essentially closed, but the Applicant has discovered that it is possible to manufacture, by pyrolysis, a carbon foam extremely rich in open micropores and mesopores. In order to develop a macroporosity sufficient to allow access of the gases to these meso- and micropores, it suffices to drill fine and closely spaced holes in the part having the geometry of the desired catalyst cut from the panel. The part is then pyrolyzed, at a temperature of 700-900 ° C., for 1 to 2 hours under a neutral atmosphere, nitrogen or argon, for example. The total specific surface of the carbon foam obtained is then greater than 300 m2 / g, that due to the mesopores being from 10 to 100 m2 / g. In these conditions, it is not necessary to activate the foam.

Polyurethane foams have multiple applications in everyday life: packaging, furniture, etc ... It is therefore very easy to get them at low cost, by recovering used foam, for example.

• découpage d'une pièce ayant la forme du support de catalyseur souhaitée;
• lavage de la mousse par une solution chaude de soude pendant quelques minutes;
• rinçage à l'eau courante et séchage;
• imprégnation de la mousse avec une résine, par exemple furfurylique et essorage de la mousse imprégnée de façon à ce qu'il reste de 0,03 à 0,1 g de résine /cm3 de mousse;
• pyrolyse de la mousse imprégnée jusque 1200°C.

The production of carbon foams from these foams involves the following steps:

• cutting of a part having the shape of the desired catalyst support;
• washing the foam with a hot soda solution for a few minutes;
• rinsing with running water and drying;
• impregnating the foam with a resin, for example furfuryl and wringing out the impregnated foam so that there remains from 0.03 to 0.1 g of resin / cm 3 of foam;
• pyrolysis of the impregnated foam up to 1200 ° C.

This procedure makes it possible to develop carbon foams having good resistance to crumbling and a homogeneous structure.

Foam activation

Carbon foams, commercial or manufactured, do not exhibit, except those prepared from PAN, a meso- and microporosity sufficient to be used as catalysts. They should therefore be activated.

To activate the foam, it is heated to a temperature between 700 ° C and 1000 ° C, under a sweep of carbon dioxide CO2 at atmospheric pressure, for a period of 1 to 24 hours.

This operation has the effect of developing, by gentle oxidation of the foam, a meso- and microporosity consisting of pores with diameters between 20 and 200 angstroms. Its specific surface thus goes from less than 1 m² / g to several tens or even hundreds of m² / g. It can be done batchwise by fixed loads or, preferably in a passage oven in which the foam parts are either placed on a conveyor belt or placed in containers which pass through the oven.

Preparation of metal carbide

Finally, the last step consists in exposing the activated foam part to the action of a volatile metal compound whose carbide is to be prepared at a temperature and for a duration such that the compound is reduced to metal, and that the metal either fueled. This step is carried out either under vacuum or under sweeping of inert gas at atmospheric pressure. The metals from which carbide is to be obtained by the process of the invention are in particular the transition metals (groups 3b, 4b, 5b, 6b, 7b, 8 of series 4, 5, 6 of the classification of elements), rare earth metals and actinides, but also the silicon.
They must also have at least one volatile compound under the temperature conditions of the reaction. Among these compounds, oxides or iodides are preferably chosen, but other compounds may also be suitable such as chlorides, fluorides or sublimable organometallic compounds. Among the metals having these characteristics and whose carbides have a catalytic interest, mention may be made of Mo, W, Re, V, Nb, Ta, Ti, Cr, Ni.

• on peut tout d'abord, comme pour l'opération précédente d'activation, opérer en discontinu en immergeant ou en posant simplement la pièce en mousse activée dans ou sur un lit de poudre du composé volatil. L'ensemble est alors placé dans une enceinte portée à la température voulue, comprise entre 1000 et 1400°C. L'application du vide ou un balayage par un gaz inerte favorise l'élimination du monoxyde de carbone CO qui se forme lorsque le composé volatil choisi est un oxyde.
• cette étape pouvant être réalisée sous balayage de gaz inerte à pression atmosphérique, on peut aussi, comme dans l'opération d'activation, travailler en continu, dans un four à passage. Les pièces de mousse activée sont toujours immergées ou posées simplement dans ou sur un lit de poudre du composé volatil. L'ensemble est placé dans des récipients successifs qui traversent le four chauffé à la température voulue et balayé par un gaz inerte.

To expose the activated foam to the action of the volatile metallic compound, several methods are possible:

• we can first of all, as for the previous activation operation, operate batchwise by immersing or simply placing the activated foam part in or on a bed of powder of the volatile compound. The assembly is then placed in an enclosure brought to the desired temperature, between 1000 and 1400 ° C. The application of vacuum or sweeping with an inert gas promotes the elimination of the carbon monoxide CO which forms when the volatile compound chosen is an oxide.
• this step being able to be carried out under sweeping of inert gas at atmospheric pressure, it is also possible, as in the activation operation, to work continuously, in a passage oven. The activated foam pieces are always submerged or simply placed in or on a bed of volatile compound powder. The assembly is placed in successive containers which pass through the oven heated to the desired temperature and swept by an inert gas.

The metallic compound can be in the form of a more or less fine powder, or in agglomerated form, granules or pellets for example.

In the particular case of silicon, it is advantageous to use as silicon volatile compound SiO. This volatile compound is obtained by reduction of the silica SiO2 either by carbon or by silicon, according to one or other of the reactions:

SiO2 + C = SiO + CO



SiO2 + Si = 2SiO

However, this sub-oxide SiO is unstable at low temperature where it undergoes a disproportionation into Si and SiO 2 according to the preceding reverse reaction. Also, when it is desired to prepare silicon carbide according to the present invention, it is necessary to manufacture the SiO in the carburetion furnace itself. This is done in a very simple way: we will proceed as in the general case described above, but we will replace the volatile compound bed with a bed of silica mixed with silicon. The reaction for transformation into SiC is rapid and complete.

The Applicant has discovered that, in the case of carburetion under an inert gas sweep, the CO content at the outlet of the reactor constituted a means of controlling its operation to ensure optimum and constant yield and good product quality.

• elle peut localement, en fonction de la température donner lieu à une rétrogradation de la réaction de carburation, le CO réagissant sur le carbure formé pour redonner de l'oxyde et du carbone, ce qui diminue d'autant le rendement du réacteur;
• elle peut donner lieu à l'oxydation de l'oxyde volatil pour former un oxyde moins volatil en générant des fines de carbone. C'est le cas, en particulier, de l'oxyde de silicium SiO qui réagit sur le CO pour redonner de la silice et du carbone. Ce dépôt de carbone sur la source de SiO (mélange de Si et de SiO2) pollue ce mélange et donne naissance à du carbure qui n'a évidemment pas les caractéristiques, notamment de surface spécifique de celui obtenu par "filiation" à partir de carbone solide introduit comme matière première. La teneur en CO à la sortie du réacteur constitue donc un moyen de piloter son fonctionnement pour assurer un rendement optimal et constant et une bonne qualité des produits, en particulier un très faible taux d'oxyde n'ayant pas réagi.

The reduction and carburetion reactions produce CO which is entrained by the inert gas, which promotes the evolution of the reaction towards the formation of carbide. The inventors have discovered the important role of the partial pressure of CO in the inert gas. If this pressure becomes too high, it has two harmful effects:

• it can locally, as a function of the temperature, give rise to a demotion of the carburetion reaction, the CO reacting on the carbide formed to give back oxide and carbon, which decreases the reactor performance;
• it can give rise to the oxidation of the volatile oxide to form a less volatile oxide by generating carbon fines. This is the case, in particular, of silicon oxide SiO which reacts on CO to restore silica and carbon. This carbon deposition on the source of SiO (mixture of Si and SiO2) pollutes this mixture and gives rise to carbide which obviously does not have the characteristics, in particular of specific surface of that obtained by "filiation" from carbon. solid introduced as raw material. The CO content at the outlet of the reactor therefore constitutes a means of controlling its operation to ensure an optimal and constant yield and good quality of the products, in particular a very low level of unreacted oxide.

This discovery led the inventors to introduce into their operating mode two additional means: the prior degassing of the carbon foam and the control of the reaction by the partial pressure of CO.

1) Prior degassing of the carbon foam.

The Applicant has shown during its research that all of the carbon monoxide present in the sweeping gas does not only come from reduction and carburation reactions, but also from the carbon foam itself. This foam, produced at a temperature lower than that of the reactions involved in the present process, contains organic chemical species on the surface, consequently comprising hydrogen and oxygen atoms. When these organic products are brought to carburetion temperature, they decompose, giving off CO and OH2, which are harmful to carburetion reactions.
One of the means of the invention therefore consists in carrying out a prior degassing of the carbon foam before bringing it into contact with the oxide of the metal from which it is desired to obtain the carbide. This degassing is carried out under primary vacuum or under the sweeping of an inert gas such as argon at a temperature and for a duration such that the carbon foam is then placed at a temperature and in a flow of inert gas identical to that of the carburetion reaction, the partial pressure of CO does not exceed, in the case of carburetion of silicon 2 hPa. This partial pressure in fact corresponds substantially to the pressure above which the reaction SiO + CO = SiO2 + C occurs in the direction of the formation of SiO2.
In practice, the recommended degassing takes place at temperatures between 1000 and 1200 ° C for periods of 0.5 to 2 hours.

2 °) Control of the reaction by the partial pressure of CO.

To allow this control, the method therefore includes a regulation loop comprising the continuous measurement or at regular intervals of the CO content of the inert gas at the outlet of the reactor, the comparison with a set interval and a modification of the operating parameters for bring the CO content within this range. It is preferable to act on the flow of inert gas which will be increased if the CO content increases. To this end, an addition of inert gas is triggered if the CO content exceeds a certain threshold, for example at 2 hPa. On the contrary, when the CO pressure becomes lower than another threshold, for example 0.05 hPa, this additional flow of inert gas is eliminated.
It is also possible to vary the reaction temperature, which is decreased or increased depending on whether the CO content increases or decreases, but this is more difficult to apply since the temperature variations have more inertia than the flow variations.

• introduire dans un réacteur un mélange réactif constitué d'une part de mousse de carbone dont la surface spécifique est au moins égale à 200 m²/g, et préalablement dégazé, d'autre part un composé, volatil dans les conditions de la réaction, du métal dont on fabrique le carbure, dans des proportions voisines de la stoechiométrie;
• balayer le réacteur par un courant de gaz inerte introduit du côté de l'entrée du mélange réactif et extrait du côté de la sortie du carbure obtenu;
• porter le mélange réactif à une température comprise entre 900 et 1400°C pendant un temps suffisant pour volatiliser le composé métallique, le réduire par le carbone et finalement carburer le produit réduit;
• refroidir le carbure obtenu, toujours en présence de gaz inerte, est refroidi jusqu'à une température telle qu'il ne s'oxyde plus au contact de l'air;
• extraire le carbure du réacteur.

et par le fait que l'on mesure en continu ou à intervalles réguliers la teneur en CO du gaz inerte à la sortie du réacteur, que l'on compare cette valeur à un interval de consigne et que l'on modifie les paramètres opératoires pour ramener la teneur en CO à l'intérieur de cet intervalle en agissant sur le débit de gaz inerte que l'on augmente si la teneur en CO augmente et que l'on diminue si la teneur en CO diminue ou sur la température de réaction.The process, including controlling the reactor by the CO content, is then characterized by the following stages:

• introducing into a reactor a reactive mixture consisting on the one hand of carbon foam whose specific surface is at least equal to 200 m² / g, and previously degassed, on the other hand a compound, volatile under the conditions of the reaction, metal from which the carbide is made, in proportions close to stoichiometry;
• sweep the reactor with a stream of inert gas introduced on the inlet side of the reactive mixture and extracted on the outlet side of the carbide obtained;
• bring the reactive mixture to a temperature between 900 and 1400 ° C for a time sufficient to volatilize the metal compound, reduce it with carbon and finally carburize the reduced product;
• cooling the carbide obtained, still in the presence of inert gas, is cooled to a temperature such that it no longer oxidizes on contact with air;
• extract the carbide from the reactor.

and by the fact that the CO content of the inert gas leaving the reactor is measured continuously or at regular intervals, that this value is compared to a set interval and that the operating parameters are modified to reduce the CO content within this range by acting on the inert gas flow rate which is increased if the CO content increases and which is decreased if the CO content decreases or on the reaction temperature.

Product obtained

The product according to the invention, that is to say the catalyst or catalyst support obtained has characteristics which reflect its filiation from carbon foam activated.
Its macroporosity thus consists of a three-dimensional network of kinds of interconnected cages whose edges have thicknesses of a few micrometers, the length of these edges being able to vary from 50 to 500 micrometers.
The mesoporosity developed by the carbon foam activation treatment remains, but the mean diameter of the mesopores has increased by an approximate factor of 3.
The microporosity developed by the carbon foam activation treatment has almost completely disappeared. This results in a BET surface area that is lower than that of the activated foam but higher than that of the starting carbon foam: it is between 20 and 100 m² / g.
The density of the carbide foam is of the same order as that of the carbon foam: from 0.03 to 0.1 g / cm³, depending on the relative dimensions of the thicknesses and the lengths of the edges. The carbide obtained consists of elementary or crystallite crystals of small dimensions, between 40 and 400 angstroms. It contains little or no residual metal, less than 0.1% by weight.

EXAMPLES. Example 1: Manufacture of carbide foam from polyacrylonitrile.

A piece having the shape of a cylinder with a diameter of 25 mm and a length of 75 mm was cut from a rigid panel of PAN. In order to develop a macroporosity sufficient to allow access of the gases to the meso- and micropores which will be developed by calcination, radial holes with a diameter of 1 mm and a center distance of 3 mm were drilled over the entire surface of the cylinder. The part was then pyrolyzed, at a temperature of 800 ° C., for 2 hours under a nitrogen atmosphere. The specific surface of the carbon foam obtained due to the micropores is then 320 m2 / g and that due to the mesopores of 40 m2 / g. There is no need in these conditions, to activate the foam.

The carbon foam is then placed in a reactor on a bed consisting of an intimate mixture of silicon powder and silica. The reactor is placed under argon sweeping with a flow rate of 50 liters / hour, then heated to a temperature of 1400 ° C. This temperature is then maintained for 5 hours. After which, still under argon sweeping, the reactor is cooled to room temperature.

The product obtained is a silicon carbide foam which is in the form of a porous texture whose macroporosity is formed by the holes drilled in the precursor, whose microporosity has practically disappeared and whose mesoporosity represents a specific surface of 35 m2 / g. The silicon carbide crystallites have small dimensions, between 40 and 400 angstroms. The product contains little or no residual metal, less than 0.1% by weight.

où
   P est la perte de charge
   l la longueur de l'échantillon
   S la section de l'échantillon
   µ la viscosité du fluide
   m la masse volumique du fluide
   d le débit de fluide
   k₁ et k₂ deux coefficients caractéristiques de la perméabilité de l'échantillon.
Il faut noter que, toutes choses égales par ailleurs, la perte de charge P est d'autant plus faible, c'est-à-dire que la perméabilité est d'autant plus élevée que k₁ et k₂ sont plus grands.The permeability of this carbide has been compared with that of a conventional catalyst support, made of cordierite, of the same shape and the same size. This is, in fact, a very important feature of use for the exhaust catalysts which must not cause too great a pressure drop in the flow at high flow rate of the exhaust gases.
The pressure drop per unit length P / l is given by the formula: $$\text{P / l = µ * d / k₁ * S + m * d² / k₂ * S²}$$

or
P is the pressure drop
l the length of the sample
S section of the sample
µ the viscosity of the fluid
m the density of the fluid
d fluid flow
k₁ and k₂ two coefficients characteristic of the permeability of the sample.
It should be noted that, all other things being equal, the pressure drop P is all the lower, that is to say that the permeability is all the higher as k₁ and k₂ are greater.

The table below gives the values of the coefficients k₁ and k₂ for conventional cordierite foam and for SiC foam. k₁ (m²) k₂ (m) Cordierite foam 9 * 10⁻⁹ 1.4 * 10⁻⁴ SiC foam 2.2 * 10⁻⁸ 1.3 * 10⁻³

It appears from this table that the coefficient k₁ is 2.4 times higher and the coefficient k₂ 9.3 times higher for SiC than for cordierite and that, consequently, silicon carbide is appreciably more permeable than cordierite .

Example 2: Manufacture of carbon foam from polyurethane.

• a) 10°C/min jusque 250°C
• b) 1°C/min jusque 800°C
• c) 3°C/min jusque 1200°C
• d) palier de 2 heures à 1200°C.

A part having approximately the shape and dimensions of an exhaust catalyst is cut from a panel of polyurethane foam. This part was washed in a 4% aqueous sodium hydroxide solution at 65 ° C for 8 minutes, then rinsed thoroughly with running water, finally dried in an oven at 150 ° C for 30 minutes or in a micro-oven. waves for 15 minutes. The dry part was then impregnated with a furfuryl resin supplemented with 2% hexamethylenetetramine and wrung out so that 0.065 g of resin / cm³ of foam remained. The impregnated foam was then pyrolyzed up to 1200 ° C. under the conditions of rise following temperatures:

• a) 10 ° C / min up to 250 ° C
• b) 1 ° C / min up to 800 ° C
• c) 3 ° C / min up to 1200 ° C
• d) 2 hour level at 1200 ° C.

The carbon foam thus obtained has a macroporosity sufficient for the access of the gases, but its mesoporosity and its microporosity are insufficient. It is therefore necessary to carry out its activation, under the conditions of example 3 below before proceeding to carburetion under the conditions of example 1.

Example 3: Activation of the foam.

We started with a commercial carbon foam of very low density: 0.03 g / cm³ whose BET surface is also very small: less than 1 m² / g.

The foam is placed in a reactor brought to a temperature of 900 ° C. in which carbon dioxide gas free of traces of oxygen is circulated, at the rate of 200 liters / hour, measured under normal conditions of temperature and pressure. The activation reaction lasts 2 hours, after which the foam is cooled to room temperature under circulation of inert gas. The activated foam obtained has a considerably increased BET specific surface area of 680 m² / g.

Example 4: Manufacture of SiC silicon carbide foam.

A sample of the activated foam obtained according to the conditions of Example 3 is placed in a reactor on a bed consisting of an intimate mixture of silicon powder and silica. The reactor is placed under argon sweeping with a flow rate of 50 liters / hour, then heated to a temperature of 1400 ° C. This temperature is then maintained for 5 hours. After which, still under argon sweeping, the reactor is cooled to room temperature.

The reaction is controlled by the partial pressure of CO in the purging gas at the outlet of the reactor: when this partial pressure exceeds 2 hPa, the flow of argon is increased; when this pressure drops below 0.05 hPa, this flow rate is reduced to its initial value.

The silicon carbide foam is then treated with air at a temperature between 600 and 800 ° C to remove the rest of the activated foam which has not reacted.

• composition de la mousse: 100 % de SiC beta; 0 % de carbone résiduel; 0 % de silicium.
• diamètre des cristallites: 52 angströms.
• surface BET:
     avant traitement d'oxydation: 19 m²/g
     après traitement d'oxydation: 22 m²/g

A silicon carbide foam is obtained having the following characteristics:

• foam composition: 100% SiC beta; 0% residual carbon; 0% silicon.
• diameter of the crystallites: 52 angstroms.
• BET surface:
  before oxidation treatment: 19 m² / g
  after oxidation treatment: 22 m² / g

Example 5: Manufacture of molybdenum carbide foam Mo₂C.

A sample of the foam obtained according to the conditions of Example 1 is placed in a reactor on a bed consisting of molybdenum oxide MoO₃. The reactor is placed under an argon sweep with a flow rate of 50 liters / hour, then heated to a temperature of 1300 ° C. This temperature is then maintained for 5 hours. After which, still under argon sweeping, the reactor is cooled to room temperature.

• composition de la mousse: 83 % de Mo₂C ; 17 % de carbone résiduel; 0 % de molybdène métal.
• diamètre des cristallites: 270 angströms.
• surface BET: 150 m²/g

We obtain a molybdenum carbide foam Mo₂C having the following features:

• foam composition: 83% Mo₂C; 17% residual carbon; 0% molybdenum metal.
• diameter of the crystallites: 270 angstroms.
• BET surface: 150 m² / g

Example 6: Comparison with silicon carbide prepared from activated carbon.

The silicon carbide foam obtained according to the conditions of Example 4 was compared with SiC granules of dimensions between 1.25 and 2.5 mm prepared from activated carbon on the one hand according to the conditions of the French application 2,621,904, that is to say under a vacuum of 10⁻² mm of mercury (1.333 Pa), on the other hand according to similar conditions in which the vacuum has been replaced by an argon sweep.

The synthesis conditions and the characteristics obtained are shown in the table below:

In this table, the sizes of the crystallites (6th column) are expressed in angström = 10⁻¹⁰ m; the 7th and 8th columns give the BET surface, expressed in m² / g, respectively before and after the oxidation treatment intended to remove the residual carbon. The composition of the phases (4th column) is obviously given before this treatment.

• les surfaces BET qui sont une caractéristique importante pour l'utilisation de ces produits comme catalyseurs ou supports de catalyseurs sont du même ordre pour les trois produits.
• l'utilisation de mousse de carbone permet d'obtenir une taille de cristallites nettement plus fine que les procédés à partir de charbon actif, qu'ils soient sous vide ou sous gaz inerte.
• la substitution d'un balayage d'argon au vide permet de réduire de façon notable les durées de traitement: de 8 à 5 heures.

The conclusions drawn from the examination of this table are:

• the BET surfaces which are an important characteristic for the use of these products as catalysts or catalyst supports are of the same order for the three products.
• the use of carbon foam makes it possible to obtain a much finer crystallite size than the processes using activated carbon, whether they are under vacuum or under inert gas.
• the substitution of an argon sweep for the vacuum makes it possible to significantly reduce the treatment times: from 8 to 5 hours.

Claims (18)

1 Heavy metal carbide or silicon foam intended in particular to serve as catalyst or catalyst support and having a large open macroporosity allowing easy access to the reaction gases, characterized in that its macroporosity is in the form of a network three-dimensional kinds of interconnected cages whose edge length is between 50 and 500 micrometers, in that its density is between 0.03 and 0.1 g / cm³, in that its BET surface is between 20 and 100 m² / g, in that the carbide foam does not contain more than 0.1% by weight of residual metal and in that the size of the carbide crystallites is between 40 and 400 angstroms. 2 heavy metal carbide or silicon foam according to claim 1 prepared from activated carbon foam characterized in that the average diameter of the mesopores of the carbide foam is approximately three times the average diameter of the mesopores of the foam of activated carbon and in that the microporosity developed by the activation treatment has almost completely disappeared. 3 Process for the preparation of heavy metal carbides or silicon with a high specific surface in the form of a monolithic part intended in particular to serve as a catalyst or a catalyst support and having a large open macroporosity allowing easy access to the reaction gases, characterized in the combination of the following successive stages: - Manufacture, from carbon foam, a part having the dimensions and the shape of the catalyst support or of the catalyst that it is desired to obtain; - activate the surface of this part by exposing it to mild oxidation by carbon dioxide CO2; exposing the part thus activated to the action of a volatile compound of the metal for which the carbide is to be prepared at a temperature between 1000 ° C and 1400 ° C, and for a period of between 2 and 6 hours, such as compound is reduced to metal, and the metal is fueled. 4 A method of preparing metal carbides with a high specific surface in the form of a monolithic part according to claim 3 characterized in that the carbon foam has a density between 0.03 to 0.1 g / cm³. 5 A method of preparing metal carbides with a high specific surface in the form of a monolithic part according to one of claims 3 or 4 characterized in that the activation of the foam by carbon dioxide takes place at a temperature between 800 and 1000 ° C for a period of between 1 and 4 hours. 6 A method of preparing metal carbides with a high specific surface in the form of a monolithic part according to claim 5 characterized in that the activation of the foam by carbon dioxide takes place in an oven with a fixed charge. 7 A method of preparing metal carbides with a high specific surface in the form of a monolithic part according to claim 5 characterized in that the activation of the foam by carbon dioxide takes place in a passage oven. 8 Process for the preparation of heavy metal carbides or silicon with a high specific surface in the form of a monolithic part intended in particular to serve as catalyst or catalyst support and having a significant open macroporosity allowing easy access to the reaction gases, characterized by the combination of the following successive stages: - Manufacture, from a rigid polyacrylonitrile panel, a part having the dimensions and the shape of the catalyst support or of the catalyst that it is desired to obtain; - drill this part of close holes of small diameter to create the open macroporosity; - pyrolyze the part, at a temperature of 700-900 ° C, for 1 to 2 hours under a neutral atmosphere; - expose the part to the action of a volatile compound of the metal whose carbide is to be prepared at a temperature between 1000 ° C and 1400 ° C, and for a period of between 2 and 6 hours, such that the compound is reduced to metal, and that the metal is fueled. 9 A process for preparing metal carbides with a high specific surface in the form of a monolithic part according to one of claims 3 to 7 characterized in that the carbon foam is produced from a polyurethane foam prepared according to the following successive steps: - washing the foam with a hot soda solution for a few minutes; - rinsing with running water and drying; - impregnating the foam with a resin and spinning the impregnated foam so that there remains from 0.03 to 0.1 g of resin / cm³ of foam; - pyrolysis of the impregnated foam up to 1200 ° C. 10 Process for the preparation of metal carbides with a high specific surface in the form of a monolithic part according to one of claims 3 to 9 characterized in that the carbon foam is subjected, immediately before the carburetion treatment, to degassing under primary vacuum or under sweeping of an inert gas such as argon, at temperatures between 1000 and 1200 ° C for periods of 0.5 to 2 hours. 11 A method of preparing metal carbides with a high specific surface in the form of a monolithic part according to one of claims 3 to 10 characterized in that the metal belongs to the group formed by the transition metals (groups 3b, 4b, 5b, 6b , 7b, 8 of series 4, 5, 6 of the classification of elements), rare earth metals and actinides. 12 A method of preparing metal carbides with a high specific surface in the form of a monolithic part according to claim 11 characterized in that the metal belongs to the group formed by Mo, W, Re, V, Nb, Ta, Ti, Cr, Ni. 13 A method of preparing metal carbides with a high specific surface in the form of a monolithic part according to one of claims 3 to 12 characterized in that the exposure of the activated part to the action of a volatile compound of the metal is done by immersing it in or placing it on a bed of the volatile metal compound. 14 Process for preparing silicon carbide with a high specific surface in the form of a monolithic part intended in particular to serve as a catalyst or a catalyst support and having a large open macroporosity allowing easy access to the reaction gases, characterized by the combination of the following steps: - Manufacture a piece of carbon foam having the dimensions and the shape of the catalyst support or of the catalyst that one wishes to obtain; - activate the surface of this part by exposing it to mild oxidation by carbon dioxide CO2; exposing the part thus activated to the action of the silicon sub-oxide SiO at a temperature and for a duration such that the sub-oxide is completely transformed into silicon carbide. 15 Process for preparing silicon carbide with a high specific surface in the form of a monolithic part according to claim 14, characterized in that the exposure of the activated carbon foam part to the action of SiO is done by immersing it in or by placing it on a bed composed of a mixture of silica and silicon. 16 A method of preparing metal carbides or silicon carbide with a high specific surface in the form of a monolithic part according to one of claims 13 or 15 characterized in that the exposure of the activated carbon foam part to the action of metallic compound or SiO is done in a fixed charge oven. 17 A method of preparing metal carbides or silicon carbide with a high specific surface in the form of a monolithic part according to one of claims 13 or 15 characterized in that the exposure of the activated carbon foam part to the action of metallic compound or SiO is made in a passage furnace. 18 A method of preparing metal carbides or silicon carbide with a high specific surface in the form of a monolithic part according to one of claims 3 to 17 characterized in that the exposure of the activated carbon foam part to the action of metallic compound or SiO is carried out under an inert gas sweep and in that the partial pressure of CO is kept below 2 hPa by increasing the flow rate of inert gas.